Cardiac imaging method may expose patients to high radiation dose

Use of the imaging technique known as cardiac computed tomography (CT) angiography (CCTA) has the potential to expose patients to high doses of radiation, and methods available to reduce radiation dose are not frequently used, according to a study in the February 4 issue of JAMA.

The 64-slice (able to scan 64 images per rotation) CCTA has emerged as a useful diagnostic imaging method for the assessment of coronary artery disease and has been proposed to be useful for evaluating patients in emergency departments with chest pain. "With the constantly increasing number of CCTA-capable scanners worldwide, the volume of CCTA scans performed is likely to show substantial further increase," the authors write. They add that the clinical usefulness of CCTA for the assessment of coronary artery disease has to be weighed against the radiation exposure of CCTA and the small but potential risk of cancer. Many clinicians may still be unfamiliar with the magnitude of radiation exposure that is received during CCTA in daily practice and with the factors that contribute to radiation dose, according to background information in the article.

Jörg Hausleiter, M.D., of the Deutsches Herzzentrum München, Klinik an der Technischen Universität München, Munich, Germany, and colleagues investigated the magnitude of radiation dose of CCTA in daily practice, factors contributing to radiation dose and the use of currently available strategies to reduce radiation dose. The trial, the Prospective Multicenter Study On Radiation Dose Estimates Of Cardiac CT Angiography In Daily Practice I (PROTECTION I), an international, multicenter study (21 university hospitals and 29 community hospitals) included 1,965 patients undergoing CCTA between February and December 2007. Analysis was used to identify independent predictors associated with radiation dose, which was measured as dose-length product (DLP; equals the average radiation dose over a specific investigated volume multiplied by the scan length), which best mirrors the radiation a patient is exposed to by the entire CT scan, according to the authors.

The researchers found that the median (midpoint) DLP of the patients in the study was 885 mGy × cm (a measurement of absorbed radiation), which corresponds to an estimated radiation dose of 600 chest x-rays. A high variability in DLP was observed between study sites (range of median DLPs per site, 331-2,146 mGy × cm).

Independent factors associated with radiation dose were patient weight (relative effect on DLP, 5 percent); absence of stable sinus rhythm (type of heart rhythm; 10 percent effect); scan length (a 1-cm increase in the scan length was associated with a 5 percent increase in DLP); the use of electrocardiographically controlled tube current modulation (resulting in a reduction of DLP of 25 percent, applied in 73 percent of patients); 100-kV (kilo volts) tube voltage (46 percent reduction of DLP, applied in 5 percent of patients); sequential scanning (78 percent reduction; applied in 6 percent of patients); experience in cardiac CT (1 percent reduction); number of CCTAs per month; and type of 64-slice CT system (for highest vs. lowest dose system, 97 percent effect).

"… the study demonstrates that radiation exposure can be reduced substantially by uniformly applying the currently available strategies for dose reduction, but these strategies are used infrequently," the authors write. "… an improved education of physicians and technicians performing CCTA on these dose-saving strategies might be considered to keep the radiation dose 'as low as reasonably achievable' in every patient undergoing CCTA."

"As CCTA is being used more frequently worldwide for diagnosing coronary artery disease, all strategies for reducing radiation exposure will finally reduce the patient's lifetime cancer risks. Although the associated risk is small (estimated lifetime attributable risk of death from cancer after an abdominal CT scan is 0.02 percent) relative to the diagnostic information for most CT studies, this risk needs to be realized especially when repeated CT scans are being performed."

(JAMA. 2009;301[5]:500-507. Available pre-embargo to the media at www.jamamedia.org)

Editor's Note: Please see the article for additional information, including other authors, author contributions and affiliations, financial disclosures, funding and support, etc.

In an accompanying editorial, Andrew J. Einstein, M.D., Ph.D., of the Columbia University College of Physicians and Surgeons, New York, writes that there are a number of implications from this study for patient care.

"First, the study results reinforce the observation that cardiac CT angiography (CTA) is still a potentially high-dose procedure, and like all procedures involving the use of ionizing radiation, a patient-specific benefit-risk analysis should always be performed to justify the imaging study. Second, the findings suggest that dose-reduction methods can be used in the majority of patients, which should serve as a wake-up call to cardiac CT laboratories that do not routinely use these methods. … Third, PROTECTION I reveals a degree of variability in radiation dose between sites that had not been previously appreciated, but which offers the potential to decrease radiation burden from cardiac CTA while maintaining diagnostic image quality by instituting quality improvement programs to close the gap. Fourth, the lack of clinically significant association between procedure volume and dose suggests that despite the general association between case volumes and quality of care, even many high-volume centers can benefit from such quality improvement programs."

"The international system of radiological protection stands on three principles: justification, optimization, and diagnostic reference levels. PROTECTION I provides valuable information pertaining to each of these in the context of cardiac CTA, and as such makes an important addition to the evidence base."

(JAMA. 2009;301[5]:545-547. Available pre-embargo to the media at www.jamamedia.org)

Editor's Note: Please see the article for additional information, including financial disclosures, funding and support, etc.